This invention relates to a transmitter for digitally processing a plurality of baseband PAM (pulse amplitude modulated) signals having a common baud or sampling rate and synchronized with one another into an orthogonally multiplexed QAM (quadrature amplitude modulated) signal and a receiver for digitally processing an orthogonally multiplexed QAM signal generated from a plurality of baseband PAM signals having a common sampling rate and synchronized with one another into replicas of the last-mentioned PAM signals. As the case may be, a transmitter and/or a receiver will be referred to as a converter in the following.
It is already known that the baseband PAM signals of the type specified above, when equal in number to an even number, are conveniently dealt with as baseband complex PAM signals, reduced to a half in number. An analog-processing type orthogonally multiplexed QAM signals transmission or communication system for such baseband complex PAM signals is effective in raising the efficiency of information transmission and is already described, for example, in an article that Burton R. Saltzberg contributed to IEEE Transactions on Communication Technology, Vol. COM-15, No. 6 (December 1967), pages 805-811, under the title of "Performance of an Efficient Parallel Data Transmission System."
Due to recent developments in digital integrated-circuit techniques and digital signal processing technology, a TDM-FDM (time-division multiplex-frequency-division multiplex) transmultiplexer for carrying out conversion between SSB (single-side-band) signals and an FDM signal in an FDM terminal station and like digital transmission systems have become known as digital-processing type FDM transmission systems. If desired, reference should be had to article contributed by Rikio Maruta and Atsushi Tomozawa to IEEE Transactions on Communications, Vol. COM-26, No. 5 (May 1978), pages 720-725, under the title of "An Improved Method for Digital SSB-FDM Modulation."
It has, however, been difficult to realize a digitally processing orthogonally multiplexed QAM signal transmission system for the baseband complex PAM signals because it is inconvenient to apply the techniques developed for a TDM-FDM transmultiplexer that inherently deals only with analog signals, such as voice signals. Furthermore, direct application of the digital signal processing to the orthogonally multiplexed QAM signal transmission system is defective, as will later be described with reference to a few of about two dozens of figures of the accompanying drawing, in that a considerable amount of digital calculation, such as digital multiplication, is necessary and in that the circuitry is accordingly complicated, particularly, because of the indispensable intricate control.